Oxide resistor materials

ABSTRACT

RESISTORS MADE FROM COMPOSITIONS COMPRISING RUTHENIUM AND/OR IRIDIUM DIOXIDES WITH FINE CRYSTALLITE SIZE ARE CHARACTERIZED BY LOW NOISE LEVELS AND LOW VALUES OF TEMPERATURE COEFFICIENT OF RESISTIVITY.

July 25,1972` y H. c. ANGUs ErAL 3,679,607

OXIDE RESISTOR MATERIALS Filed Oct. 19. 1967 l 2 Sheets-Sheet 1 g Il.

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' oxIDE REsIsfrR IJATERIAL` l2 Sheets-Sheet 2 Filed oct. 19. 1967 WWW HU( WH United States Patent O 3,679,607 OXIDE RESISTOR MATERIALS Hamish Carmichael Angus, High Wycombe, and Peter Edward Gainsbury, Ruislip, England, assignors to The International Nickel Company, I nc., New York, N.Y.

Filed Oct. 19, 1967, Ser. No. 676,515 Claims priority, application Great Britain, Oct. 24, 1966, 47,690/ 66 Int. Cl. H01b 1/02 U.S. Cl. 252-518 15 Claims ABSTRACT F THE DISCLOSURE Resistors made from compositions comprising ruthenium `and/or iridium dioxides with fine crystallite size are characterized by low noise levels and low values of temperature coeicient of resistivity.

The present invention relates to electric resistance elements and more particularly to oxide resistors and to compositions for making oxide resistors.

Resistors consisting of a film of resistance material fused to a refractory, non-conducting base are known. The non-conducting base may be glass or ceramic and the resistance material a mixture of conducting and nonconducting materials. The resistance material has usually comprised specially treated metal powders and glass particles and the film has been formed by applying these powders and particles to the base as a dispersion in an organic vehicle by dipping, brushing, spraying or silkscreen printing and has subsequently been heat-treated to fuse it to the base. However, severe difiiculties in the way of obtaining consistent results have arisen when such prior means were employed in efforts to consistently produce resistors having resistances greater than about 30,000 ohms per square. To overcome such difficulties and limitations it has been proposed to replace the metal powders by an oxide of ruthenium or iridium or by more than one such oxide, for instance as described in U.S. Pat. No. 3,304,199.

The resistance of films containing an oxide, or oxides, for the electrically conducting phase decreases as the proportion of oxide increases. By appropriate choice of the proportions of the oxide and the glass, films may consistently be prepared having resistances ranging 'from zero and then through increasing positive values. Thus while the TCR of films of low oxide content having resistances in the upper part of the above-mentioned range may be satisfactory, it in some instances becomes unacceptably high and positive at resistances below about 1500 ohms per square.

In addition to needs for obtaining consistent resistance levels and low TCR values, there also are needs for obtaining low levels of current noise and it has been highly desirable to produce resistors, including resistance films, characterized by advantageously low levels of current noise in combination with small TCR values. Although many attempts were made to overcome the foregoing difficulties and other difficulties and disadvantages, none, as far as we are aware, was entirely successful in consistently overcoming all these difficulties and disadvantages whenV carried into practice commercially on an industrial scale.

It has now been discovered that new and highly desirable combinations of oxide resistor characteristics, including small TCR values and low noise levels, are ob- I 3,679,607 Patented July 25, 1972 ICC tained through special control of particle sizes and crystallite sizes of resistor oxides.

It is an object of the present invention to provide an oxide resistor'.

A further object of the invention is to provide a composition for making oxide resistors.

Another object of the invention is to provide a process for Iproduction of oxide resistors.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIGS. 1, 2, and 3 are charts depicting electrical characteristics of resistance, temperature coefficient of resistance and current noise, respectively, pertaining to resistors made with ruthenium dioxide compositions according to the invention and to other resistors made with ruthenium dioxide compositions not in accordance with the invention.

FIG. 4 is a plan view of an oxide resistor of the invention; and i IFIG. 5 is a cross sectional view of the resistor illustrated in FIG. 4.

Generally speaking the present invention contemplates a composition which is adapted to be applied to a refractory, non-conducting base and fired to form an electrical resistance and which comprises ruthenium dioxide or iridium dioxide or both having a crystallite size up to and not greater than about 500 angstroms (A.) and advantageously not -greater than 300 A., in admixture with glass, the proportion of oxide being up to about of the mixture. This invention especially involves the surprising discovery that the TCR of an oxide-glass resistance film of a given composition depends largely upon the crystallite size of the oxide, being more negative and thus less positive for smaller crystallite sizes. The absolute value of the TCR of films having a positive TCR can thereby be reduced. Resistances made from compositions in accordance with the invention have TCR values more negative than those made from the oxides available commercially and characterized by much larger crystallites.

A further yadvantage of having the oxide particles of crystallite sizes not greater than 500 A. is that the current noise of the resistance film is significantly lower than with oxides with larger crystallites.

There may `be as little as 2% of the dioxide in the mixture but usuallythere is 10% or more. Since the resistance ofV a film produced from the composition decreases as the proportion of oxide increases, it is for most purposes unnecessary to employ more than 60% by Weight of dioxide.

To some extent the TCR also depends on the size of the discrete particles of the oxide, having smaller positive 'values for smaller particles. To obtain the lowest values of TCR the particles should therefore be very line, the average particle size desirably being 0.2 to 3 microns. The size of the non-conducting glass particles is not critical. An average size of about 3.5 microns is satisfactory for the glass particles.

The composition is advantageously provided as a suspension of the mixture of oxide and glass in a liquid vehicle that is suitable for application to the refractory base by screen printing or other convenient methods. The invention includes resistances made by the use of the composition.

One method according to the invention of making ruthenium dioxide with the requisite crystallite size is as follows. Ruthenium chloride solution containing approximately 5 grams per liter (g./l.) ruthenium is treated with sodium hydroxide until it is just alkaline, and then acidied with hydrochloric acid to pH 6-7. The resultant suspension of hydrated dioxide is washed several times with distilled water by decantation, then filtered on.

a Buchner funnel and washed with hot distilled water until it is free of sodium chloride. Alternatively, after the initial washing by decantation, the suspension of hydrated dioxide is dialysed in an ion-selected membrane cell until the aqueous phase shows minimum conductivity (corresponding to the presence of less than parts per million sodium chloride), and the suspension of dioxide is then filtered. In either case the ltered dioxide is oven-dried at 110 C.

The dioxide thus produced is in the hydrated form and can be used in this form or can be rst heated to expel the water of crystallization. It has been found that the crystalline size increases if the dioxide is heated to too high a temperature and if this excess 500 C. there is a risk that the crystallite size will exceed 500 A.

Although the invention is not limited to ruthenium dioxide produced by the method described above, some other methods are found to lead to unsatisfactory high average crystallite sizes of 1000 A. or more. These unsatisfactory methods include heating ruthenium powder in air and precipitating ruthenium dioxide from a solution of sodium ruthenate. A typical commercially-avaih able RuO, powder has an average particle size of about 7 microns, i.e., 70,000 angstroms. v

One method of producing iridium dioxide of therequisite crystallite size comprises adding sodium bromate to iridium chloride solution, adjusting the pH to 7 by the addition of sodium carbonate, and boiling the solution for one hour. Hydrated iridium dioxide is precipitated and is liltered, washed free of the ,chloride and dried. The hydrated oxide is then dehydrated by heating in air for one hour at 700 C. and ground. In this heating step it is important not to go too high a temperature, in order to avoid the risk of excessively in creasing the crystallite size.

It is desirable at least not to exceed 800 C. prior to firing the resistors. t

lParticles of a batch (Batch I) of ruthenium dioxide (Oxide A), produced according to the aforedescribed method comprising treatment of ruthenium chloride with sodium hydroxide, acidification, ltration and washing, when examined in an electron microscope were found to contain well-defined crystallites, which usually took an irregular, basically hexagonal, shape but often approached a spheroidal form. Another batch of ruthenium dioxide (Batch II of Oxide B) was produced by the same method and small crystallite sizes in somewhat differently sized particles were obtained.

,Portionsv of Batch I and of Batch II were separately mixed with particles of a lead-borosilicate glass of the compositori `65% PbO, 25% Si02, 10% B303 of average size 3.5 microns in the proportion of 25% dioxide to 75% glass. 'I'he mixtures were suspended in liquid mediums consisting, by weight, of 2% ethyl cellulose and 10% of a dispersing agent, the balance being terpineol, to form resistor inks, each of which was screenprinted on to a base of the refractory material forsterite (MgZSiO), dried at 150 C. for one hour and rcd for one hour at 850 C. to form resistors having an average film thickness of 10 microns and a resistance of 1500 ohms per square. A ruthenium dioxide Batch III was also made by the method of making Batches I and II and dioxides from a portion of Batch III and from another portion of Batch II were each heated to 500 C. until the water of crystallization was removed, thereby providing Oxides C and D, respectively, which were similarly formed into resistors.

Particles of iridium dioxide (Oxide E) produced from iridium chloride according to the aforedescribed method were similarly formed into a resistor.

By way of comparison with the foregoing ive examples of the invention, the following table'includes corresponding data pertaining to the following: Oxide F: Additional portions of the same batch o (Batch I) ruthenium dioxide after being heated to Oxide G: Ruthenium dioxide produced by heating ruthenium chloride in air for 1 hour. A Y A Oxide H: Ruthenium dioxide produced by heating ruthenium powder in air.

Oxide I: Ruthenium dioxide precipitated from sodium ruthenate solution with methyl alcohol and heated to 500 C.- Crystallite sizes and particle sizes of Oxides A through E, which are in accordance with the invention, and of Oxides F through I, which are not in accordance with the invention, are setforth in -the following table along with TCR valuesA pertaining to resistors made with the corresponding oxides. Crystallite sizes are the average of the largest and smallest dimensions or the averagediameters of the spherical crystallites.

TABLE Particle size (microns) NOTEr-Partiele size=Average particle size o the dioxide in microns, TCR (p.p.m./ C.)=Temperature eoetlicient ot resistance in parts million per C. for a resistor ol 1,500 ohms per square over the range of 20C.to1250. y Crystallite sizes were measured -with an electron microscope and particle sizes were determined by the Fisher sub-sieve sizing technique. v

It will be seen that the TGR remained at a ylow value when the crystallite size was V50-300 A. even when the particle size was as high as 1 micron, and that the eiect of variation of the particle size was very small compared with that of varying the crystallite size. f

e -Additional resistors were prepared using the ruthenium dioxides, Oxide B and Oxide H and varying the proportions of oxide and glass, all other variables being kept constant. The resistance, TCR and rcurrent noise were measured for each of theresistances by standard techniques, and the results are illustrated graphically in the accompanying drawing, the variation of resistance with ruthenium dioxide content of the film being shown in FIG. 1, the vvariation of theTCR with ruthenium dioxide content being shown in FIG. 2 and the variation of the current noise with ruthenium dioxide content being shown in FIG. 3.

It will be seen from FIG. 1 that the resistance decreases progressively as the proportion of oxide increases inasmuch as -the points `for resistors made from Oxide B and Oxide H lie on the same curve (Curve BH1). It is thus apparent that for a given composition the resistance is substantially independent of the crystallite size of the oxide used. r l

FIG 2 shows that Curve B-2, which relates TCR and composition for the resistances according to the invention made from Oxide B, lies wholly below Curve H-Z that relates TCR and composition of resistances made from Oxide H, which was of larger crystallite size. It is preferred in practice that lm resistors should have'a TOR not exceeding 300 p.p.m./ C., and it is apparent that by means of the invention this can be achieved down to much lower values of the resistance than in the case of resistors made from commercially-available oxides.

In FIG. 3 the values of current noise in decibels are plotted as ordinates. The reduction in noise achieved by means of the invention is clearly evident for widely varying proportions of oxide, as illustrated by Curves B3 and H-3 with the points along Curve B3 pertaining to resistances according to the invention made from Oxide B and with the points along Curve H-3 pertaining to resistances made from Oxide H.

It will be appreciated that the value of the TCR of an oxide-glass resistance lm of a given composition will also be alfected, in a manner well understood by those skilled in the art, by other factors, including the nature of the glass and the refractory base, the firing schedule and the thickness of the lm. While these variables have been kept constant in the comparative tests referred to in the foregoing, it is of course necessary to take them into account in carrying the invention into practice to produce additional resistors having desired characteristics referred to herein. n

Referring again to the drawing, FIG. 4 shows fired resistor 1, made using a ruthenium dioxide composition in accordance with the invention, fused on ceramic substrate 2 between conductive leads 3 and 4. FIG. 5 is a cross sectional view, along the section 5 5 shown on FIG. 4, illustrative of ruthenium dioxide particles 6 in glass matrix 7. It is to be understood that inasmuch as the crystallites may grow during ring of the resistor, the initial crystallite size of the dioxide in the composition prior to firing is not per se necessarily a characteristic of the red resistor even though engendering highly important results in the nal product.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims. It will of course be understood by those skilled in the electronic arts that the present invention is particularly applicable to the production of highly stable, low noise resistors useful in electric circuits, including printed circuits, for electronic components and apparatus.

We claim:

1. A composition adapted to be appliedto a refractory, non-conducting base and red to form an electrical resistance comprising a mixture of powdered glass and about 2% to about 90% of dioxide selected from the group consisting of ruthenium dioxide, iridium dioxide and mixtures thereof with said dioxide having a crystallite size up to about 500 angstroms.

2. A composition as set forth in claim 1 wherein the dioxide component consists essentially of ruthenium dioxide.

3. A composition as set forth in claim 1 wherein the dioxide component consists essentially of iridium dioxide.

4. A composition as set forth in claim 1 wherein the proportion of dioxide is about 10% to about 60%.

5. A composition as set forth in claim 1 wherein the dioxide and glass are suspended in a liquid medium.

6. A composition as set forth in claim 1 wherein the crystallite size of the dioxide does not exceed 300 angstroms.

7. A composition as set forth in claim 6 wherein the proportion of dioxide is about 10% to about 60%.

8. -A composition as set forth in claim 6 wherein the dioxide and glass are suspended in a liquid medium.

9. A resistor comprising a non-conducting base having fused to its surface an electrically resisting film formed from the composition set forth in claim 1.

10. A resistor comprising a non-conducting base having fused to its surface an electrically resisting lm formed from the compostion set forth in claim 6.

11. In a process wherein a composition containing metal oxide particles and glass particles is applied to a substrate and red to produce a resistance element from the glass and metal oxide composition, the improvement comprising providing in said composition dioxide particles having a ne crystallite size of up to about 500 angstroms rand selected from the group consisting of ruthe'nium dioxide, iridium dioxide and mixtures thereof.

12. A process as set forth in claim 11 wherein dioxides are provided with a crystallite size that does not exceed 300 angstroms.

13. A process as set forth in claim 11 wherein the size of the dioxide particles is controlled to provide that the average particle size of the dioxide is 0.2 to l micron.

14. A process as set forth in claim 11 wherein the dioxide is prepared by precipitation from a chloride solution and wherein any heating of the dioxide prior to firing the resistance element is controlled to avoid exceeding 800 C. and is further controlled to avoid exceeding 500 C. when the composition contains ruthenium dioxide.

15. A composition as set forth in claim 1 wherein the average particle size of the dioxide is 0.2 to 1 micron.

References Cited UNITED STATES PATENTS 2,950,996 8/ 1960 Place, Sr. et al 252-514 3,304,199 2/1967 Faber, Sr. et al 252--514 3,324,049 6/ 1967 Holmes 252-514 3,329,526 7/1967 Daily et al 106-47 3,343,985 9/1967 Vickery 252-1514 3,450,545 6/ 1969 Ballard 117-229 DOUGLAS I. DRUMMOND, Primary Examiner 

